A main shaft of a turbo-machine (for example, a gas turbine or a turbocharger) is rotated at high speed under a high-temperature environment. Further, there is a case where the turbo-machine has a difficulty in being provided with a separate auxiliary machine for oil circulation from the viewpoints of energy efficiency, and there is also a case where shear resistance of a lubricating oil may be a factor for inhibiting high-speed rotation of the main shaft. Therefore, as a bearing for supporting the main shaft of the turbo-machine, an air dynamic pressure bearing is often used in place of a rolling bearing or a dynamic pressure bearing with oil lubrication.
In the air dynamic pressure bearing, both a bearing surface on a rotary side and a bearing surface on a stationary side are generally constructed of rigid bodies. However, in this type of air dynamic pressure bearing, when management of a gap width of a bearing gap formed between both the bearing surfaces is insufficient, self-excited whirling called a “whirl” becomes more liable to occur in the main shaft upon exceeding a stability limit. Thus, in a general air dynamic pressure bearing, the gap width of the bearing gap needs to be managed with high accuracy in order to stably exhibit desired bearing performance. However, under an environment accompanying with a large temperature change as in the turbo-machine, the gap width of the bearing gap is easily varied under the influence of thermal expansion. Therefore, there is difficulty in stably exhibiting the desired bearing performance.
There has been known a foil bearing as a bearing which is less liable to cause the whirl and enables management of a gap width of a bearing gap to be performed easily even under the environment in which a temperature change is large. The foil bearing has a bearing surface which is constructed of a flexible metal thin plate (foil) having low rigidity against bending. The foil bearing allows flexure of the bearing surface, to thereby support a load. For example, in Patent Literature 1 described below, there is described one type of a foil bearing configured to support a radial load.
In the foil bearing of Patent Literature 1, when a rotary shaft is rotated, an air film is formed in a radial bearing gap between a radially inner surface of a top foil (bearing foil) constructing a stationary side and an outer peripheral surface of the rotary shaft opposed to the radially inner surface of the top foil, and the rotary shaft is supported in a radial direction by the pressure of the air film. Further, in this foil bearing, the top foil and an elastic support portion (elastic body) configured to elastically support the top foil are elastically deformed in accordance with operation conditions such as a load acting on the top foil and ambient temperature, to thereby automatically adjust the gap width of the radial bearing gap. Therefore, the foil bearing has an excellent feature in stability as compared to a general dynamic pressure bearing, and can be suitably used as a bearing for supporting a rotary body which is rotated at high speed under the high-temperature environment, such as a rotor of the turbo-machine.
Further, in a general dynamic pressure bearing, the gap width of the radial bearing gap needs to be managed on the order of 1/1,000 of a shaft diameter. Thus, for example, when the general dynamic pressure bearing is used for supporting the rotary shaft having a diameter of about several millimeters, the gap width of the radial bearing gap needs to be managed to about several micrometers. However, when tolerance or thermal expansion amount at the time of manufacturing is taken into consideration, management of the gap width on the above-mentioned order is not easy to perform. In contrast, with the foil bearing, the gap width of the radial bearing gap is automatically adjusted through elastic deformation of the top foil (bearing surface) itself, and hence the gap width of the radial bearing gap only needs to be managed to about several tens of micrometers. Thus, the foil bearing also has an advantage in that manufacture of the foil bearing and management of the gap width of the bearing gap can be performed more easily as compared to the general dynamic pressure bearing.
The above-described advantages of the foil bearing can be similarly found in a type of foil bearing configured to support a thrust load.
Incidentally, particularly during low-speed rotation of the rotary shaft, rigidity (pressure) of the air film formed in the bearing gap is not sufficiently increased, and hence the bearing surfaces repeatedly come into slide contact with each other. In order to prevent abrasion of the bearing surfaces and an increase in rotation torque due to such slide contact to the largest extent possible, there has also been conceived to provide, on at least one of the bearing surfaces, a film exhibiting excellent abrasion resistance such as nickel plating or chromium plating, or a film exhibiting good lubricity such as a plating film or a resin coating in which a solid lubricant made of molybdenum disulfide, graphite, fluororesin, or the like is dispersed (see Patent Literature 1).